Today Weather's Discussion: Weekend Details
Thursday, Dec. 9:
Yesterday I wrote that I wanted to hold off on showing you maps for the weekend due to model inconsistencies, and while I'm still not extremely confident with the models yet, if it's maps you want...it's maps you'll get today.
First, a quick word on the near-term forecast. A minor system moving across the upper Midwest today will keep clouds around tonight, which will help keep overnight lows up a few degrees (upper 20's to near 30). Clouds should depart for a while in the morning, giving us some peaks of sunshine during the first half of the day, before clouds roll back in Friday afternoon ahead of the weekend system.
On to the weekend...let me first say that any forecast is still subject to change and will need fine-tuning over the next couple of days. Forecast models have shown little consistency over the past few days with regards to the track of this system, and that makes putting forth a high-confidence forecast rather difficult. The upper level low that will give rise to this weekend's system is still over the eastern Pacific Ocean, and forecast models usually do not lock onto a more definite solution until the storm arrives onshore and can be better looked at by weather observation units. The system should move onshore tonight, so by tomorrow morning, I'm confident that we'll be able to start getting a little more certain on things.
Forecast models have trended toward suggesting a more northerly track to this system, and the farther north it goes, the lesser our chances of snow. In fact, the latest round of models all lock on to the northern track. There are still some inconsistencies with regards to exactly how fast the cold air arrives Sat. night and how much rainfall we'll get, but those are details I suspect will become clearer by tomorrow.
Here's a look at a few things I looked at this morning....first of all, how much rain to expect? Big differences between the GFS, which is showing barely 0.1" in it's latest run, compared to the NAM, as shown below, squeezing out some decent rainfall Saturday night...perhaps on the order of an inch or more:
By the way, on the image above, you will notice a thin, solid, yellow line running from near Chicago, back toward St. Louis, and back near Springfield, MO. This is know as the "540 line", and acts as a rough estimate of the rain/snow line. This would obviously mean all rain for us at 6PM Saturday.
The tricky parts are the questions of when does the cold air arrive & when does the moisture move out? In looking at the GFS, it appears that a wedge of drier air, commonly referred to as the "dry slot", will spell a temporary end to precip just as the cold air that would support snow moves in. Check out the images below....
The image above shows temperatures at about 1500 feet, with the cold air making its arrival at around 6PM Saturday. This shows temperatures diving below the freezing mark at that level of the atmosphere. Let's move on to another image, also at 6PM Saturday, showing that dry slot that I just mentioned:
Notice the wedge of purple that is shown over southern MO and into central IL? That is an area of drier air being pulled into the center of the storm. The deeper moisture necessary for precipitation is both east and north of us, just as the cold air arrives. So what does that mean? If that solution occurs, we'll see rain temporarily ending Saturday evening as cold air arrives....meaning any changeover to snow would be very light.
As the system moves east and northeast into Sunday, the moisture showing up above across parts of Iowa, northern IL, and northern MO will likely wrap around the low and spill south across our area, leading to some snow showers on Sunday. If models continue to come back drier, I wouldn't be surprised if those snow showers don't amount to much.
So as far as accumulation.....It looks highly unlikely that we'll see much of anything, aside from perhaps around I-64 in IL.
This is a map showing the HPC/NWS snowfall probability through 6AM Sunday, showing less than a 30% chance of 1" or more of snow. Chances of 1" or more get into the 50-60% range closer to I-70 in MO & IL.
This is probability of 2" or more through 6AM Sunday, with our area shown at less than 10%. Notice that those maps only go through 6AM Sunday.....so there could be some minor additional accumulations unaccounted for by these maps during the day Sunday.
So, bottom line on the weekend.....rain looks like a good bet Saturday, although with models trending drier, I wouldn't be surprised if we see more in the way of showers instead of an all-day rain. Saturday night, some light rain changing to snow, and some snow showers Sunday. Maybe a dusting of snow here and there, and perhaps up to an inch north of Route 13 in IL.
Sunday will be very windy as well, with NW wind 15-25 mph, gusting to 30 mph. This will mean wind chills in the teens. Lows Sunday night fall back to the mid teens, with wind chills in the single digits. Things stay cold Monday and into Tuesday, with highs Monday in the mid 20's, and lows Monday night in the mid teens again. We'll finally start to see some warmer temperatures arrive by next week's second half.
We'll still be keeping a close eye on the weekend system. Be sure to tune into Local 6 for the latest updates!